PQ Comes in Twos

The acronym “PQ” most often is associated with power quality, but some people use it to represent power quantity. The latter is more commonly referred to in today’s technical community as energy management. The commonality between the two is further seen in the instruments used to measure such.

Twenty years ago, there were two different instruments used: one for quality and one for quantity. Now, most power quality monitors provide the parameters used in energy management (real or active power, apparent power, reactive power, power factor, etc.). Conversely, some energy meters measure the basic set of power quality parameters (sags, swells, harmonic distortion, etc.). However, providing the complete set of power quality phenomena monitoring is beyond the capability of nearly all of the energy meters with PQ, and few power quality monitors provide the certified power measurement accuracy of the energy meters.

The overlap continues with some of the devices used to save energy. Some of them significantly reduce the quantity consumed but do so at the expense of decreasing the quality. For example, adjustable speed drives are a significant contributor to the reduction in electricity consumed by electric motors, which are considered in most surveys to be the largest consumer of the electrical energy (as compared to other categories such as lighting). They create this savings by consuming only as much energy as the load requires at that time. The front end of such devices has a power converter, changing the AC voltage into DC voltage and then back into AC at the frequency required to provide the necessary energy to the load. For example, rather than turning a ventilation fan at full speed when the room is at the proper temperature, according to the thermostat, the rotation speed (which is a function of the frequency of the voltage, not the amplitude) is reduced. Or, there also are pulse-width-modulated drives, where the width of the pulses—which translates into amount of energy—is reduced when less work is required of the motor, just enough to keep it rotating at the proper speed. The simplest example of this is probably the single-phase dimmer switch with its partial waveform of current.

These power converters are nonlinear loads by the nature of how they conserve energy. They draw energy only during part of the power frequency cycle, so the voltage and current waveforms are not similar to each other. This is the result of the harmonics in the waveforms. Remember, harmonics are one of the most common parameters used to determine the quality of the electrical supply. With the case of three-phase, full-wave rectifiers, the most common harmonics will be the fifth and seventh and the 11th and 13th. This is in contrast to the single-phase converters, such as used with PCs and other IT equipment, where the energy consumed is less than an equivalent linear-type power supply, but there are significant third, fifth and seventh current harmonics resulting from such.

Another energy conservation program that impacts power quality is in the energy-efficient lighting devices. Though they are much improved from the past, the electronic ballasts that replaced the magnetic ballasts for fluorescent lighting can be significant harmonic contributors. Likewise, there has been a recent move to replace incandescent bulbs with compact fluorescent light bulbs, with some countries stating obsolescence mandates of incandescents in the near future. Once again, no free lunch. The quantity can be at the expense of the quality, with some potential voltage transients during startup as well on ongoing harmonics. In addition, there was a recent article on the Internet indicating that a person who broke a CFL bulb on the bathroom floor spent more than $2,000 to remediate the mercury contamination from it.

On the flip side, there have been claims of power quality mitigation equipment being able to save energy, such as transient voltage surge suppressors (the outlet strips into which people plug their PC, printer, scanner and coffee pot) and harmonic filters. Unless these devices are employed on the utility side of where the revenue meters are, it is unlikely that they are saving the consumer any energy costs.

Diverting voltage transients or harmonic currents from getting further downstream to the loads does not mean that the shunted, clamped or filtered energy did not pass through the revenue meter. In the case of the transient, it probably is a moot point anyway, since the magnetic spinning disk type of revenue meter would not have responded and measured the energy in that high frequency band anyway.

Just to be clear—this is not an anti-green, energy-guzzling-condoning article. Rather, the user of energy-efficient devices should be aware that the use of such can result in a conflict between the two PQs. In some cases, it may have no adverse effect on the ability of the facility to keep its production running smoothly. But sometimes it will, and you should be especially aware if equipment starts to malfunction after other equipment was changed out for a more energy-efficient model. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

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